FEC Framework M. Watson
Internet-Draft Digital Fountain
Intended status: Standards Track July 7, 2008
Expires: January 8, 2009
Raptor FEC Schemes for FECFRAMEdraft-watson-fecframe-raptor-00
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Abstract
This document describes Fully-Specified Forward Error Correction
(FEC) Schemes for the Raptor code and its application to reliable
delivery of media streams in the context of FEC Framework. The
Raptor code is a systematic code, where a number of repair symbols
are generated from a set of source symbols and sent in one or more
repair flows in addition to the source symbols that are sent to the
receiver(s) within a source flow. The Raptor code offers a close to
optimal protection against arbitrary packet losses at a low
computational complexity. Two FEC Schemes are defined, one for
protection of arbitrary packet flows and another for protection of a
single flow that already contains a sequence number. Repair data may
be sent over arbitrary datagram transport (e.g. UDP) or using RTP.
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Internet-Draft Raptor FEC Scheme July 20081. Introduction
The FEC Framework [I-D.ietf-fecframe-framework] describes a framework
for the application of Forward Error Correction to arbitrary packet
flows. Modelled after the FEC Building Block developed by the IETF
Reliable Multicast Transport working group ([RFC5052]), the FEC
Framework defines the concept of FEC Schemes which provide specific
Forward Error Correction schemes. This document describes two FEC
Schemes which make use of the Raptor FEC code as defined in
[RFC5053].
The FEC protection mechanism is independent of the type of the source
data, which can be an arbitrary sequence of packets, including for
example audio or video data. In general, the operation of the
protection mechanism is as follows:
o The sender determines a set of source packets to be protected
together based on the FEC Framework Configuration Information.
o The sender arranges the source packets into a set of source
symbols, each of which is the same size.
o The sender applies the Raptor protection operation on the source
symbols to generate the required number of repair symbols.
o The sender packetizes the repair symbols and sends the repair
packet(s) along with the source packets to the receiver(s).
Per the FEC Framework requirements, the sender MUST transmit the
source and repair packets in different source and repair flows,
respectively. At the receiver side, if all of the source packets are
successfully received, there is no need for FEC recovery and the
repair packets are discarded. However, if there are missing source
packets, the repair packets can be used to recover the missing
information.
The operation of the FEC mechanism requires that the receiver can
identify the relationships between received source packets and repair
packets and in particular which source packets are missing. In many
cases, data already exists in the source packets which can be used to
refer to source packets and to identify which packets are missing.
In this case we assume it is possible to derive a "sequence number"
directly or indirectly from the source packets and this sequence
number can be used within the FEC Scheme. This case is referred to
as a "single sequenced flow". In this case the FEC Source Payload ID
defined in [I-D.ietf-fecframe-framework] is empty and the source
packets are not modified by the application of FEC, with obvious
backwards compatibility advantages.
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Otherwise, it is necessary to add data to the source packets for FEC
purposes in the form of a non-empty FEC Source Payload ID. This case
if referred to as the "arbitrary packet flow" case. Accordingly,
this document defines two FEC Schemes, one for the case of a single
sequenced flow and another for the case of arbitrary packet flows.
2. Document Outline
This document is organised as follows:
Section 5 defines general procedures applicable to the use of the
Raptor code in the context of the FEC Framework.
Section 6defines an FEC Scheme for the case of arbitrary source
flows and follows the format defined for FEC Schemes in
[I-D.ietf-fecframe-framework]. This scheme is equivalent to that
defined in [3GPP MBMS Specification].
Section 7 defines an FEC Scheme similar to that defined in
Section 6but with optimisations for the case where only limited
source block sizes are required. This scheme is equivalent to
that defined in [DVB AL-FEC specification] for arbitrary packet
flows.
Section 8 defines an FEC Scheme for the case of a single sequenced
flow and follows the format defined for FEC Schemes in
[I-D.ietf-fecframe-framework]. This scheme is equivalent to that
defined in [DVB AL-FEC specification] for the case of a single
sequenced flow.
3. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. Definitions and Abbreviations
The definitions, notations and abbreviations commonly used in this
document are summarized in this section.
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Internet-Draft Raptor FEC Scheme July 20084.1. Definitions
This document uses the following definitions. For further
definitions that apply to FEC Framework in general, see
[I-D.ietf-fecframe-framework].
Source Flow: The packet flow(s) carrying the source data and to
which FEC protection is to be applied.
Repair Flow: The packet flow(s) carrying the repair data.
Symbol: A unit of data. Its size, in bytes, is referred to as the
symbol size.
Source Symbol: The smallest unit of data used during the encoding
process.
Repair Symbol: Repair symbols are generated from the source symbols.
Source Packet: Data packets that contain only source symbols.
Repair Packet: Data packets that contain only repair symbols.
Source Block: A block of source symbols that are considered together
in the encoding process.
FEC Framework Configuration Information: Information that controls
the operation of the FEC Framework. Each FEC Framework instance has
its own configuration information.
FEC Payload ID: Information that identifies the contents of a packet
with respect to the FEC scheme.
Source FEC Payload ID: An FEC Payload ID specifically used with
source packets.
Repair FEC Payload ID: An FEC Payload ID specifically used with
repair packets.
4.2. Abbreviations
o FSSI: FEC-Scheme-Specific Information.
o SS-FSSI: Sender-Side FEC-Scheme-Specific Information.
o RS-FSSI: Receiver-Side FEC-Scheme-Specific Information.
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Internet-Draft Raptor FEC Scheme July 20085. General procedures for Raptor FEC Schemes
This section specifies general procedures which apply to all Raptor
FEC Schemes, specifically the construction of source symbols from a
set of source transport payloads. As described in
[I-D.ietf-fecframe-framework] for each source transport payload in a
source block, the FEC Scheme is provided with:
o A description of the source transport flow with which the
transport payload is associated and an integer identifier
associated with that flow.
o The source transport payload itself.
o The length of the source transport payload.
For each source transport payload, we define the Source Packet
Information (SPI) as follows:
Let
n be the number of source transport payloads in the source block.
T be the source symbol size in bytes. Note: this information is
provided by the FEC Scheme as defined below.
i the index to the (i+1)-th source transport payload to be added
to the source block, 0 <= i < n.
R[i] denote the number of octets in the (i+1)-th source transport
payload.
l[i] be a length indication associated with the i-th UDP packet -
the nature of the length indication is defined by the FEC Scheme.
L[i] denote two octets representing the value of l[i] in network
byte order (high order octet first) of the i-th UDP packet.
f[i] denote the integer identifier associated with the source
transport payload from which the i-th source transport payload was
taken.
F[i] denote a single octet representing the value of f[i].
s[i] be the smallest integer such that s[i]*T >= (l[i]+3). Note
s[i] is the length of SPI[i] in units of symbols of size T bytes.
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P[i] denote s[i]*T-(l[i]+3) zero octets. Note: P[i] are padding
octets to align the start of each UDP packet with the start of a
symbol.
SPI[i] be the concatenation of F[i] ,L[i], R[i] and P[i].
Then, a source data block is constructed by concatenating SPI[i] for
i = 0, 1, 2, ... n-1. The source data block size, S, is then given
by sum {s[i]*T, i=0, ..., n-1}. Symbols are allocated integer
Encoding Symbol IDs consecutively starting from zero within the
source block. Each source transport payload is associated with the
Encoding Symbol ID of the first symbol containing SPI for that
packet. Thus, the Encoding Symbol ID value associated with the j-th
source packet, ESI[j], is given by ESI[j] = 0, for j=0 and ESI[j] =
sum{s[i], i=0,...,(j-1)}, for 0 < j < n.
Source blocks are identified by integer Source Block Numbers. This
specification does not specify how Source Block Numbers are allocated
to source blocks. The Source FEC Packet Identification Information
consists of the identity of the source block and the Encoding Symbol
ID associated with the packet.
6. Raptor FEC Scheme for arbitrary packet flows6.1. Introduction
This section specifies an FEC Scheme for the application of the
Raptor code to arbitary packet flows. This scheme is recommended in
scenarios where maximal generality is required.
This scheme is equivalent to that specified in [3GPP MBMS
Specification].
6.2. Formats and Codes6.2.1. FEC Framework Configuration Information6.2.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the fully-specified FEC scheme
defined in this section MUST be TBD as assigned by IANA.
6.2.1.2. Scheme-Specific Elements
The scheme-specific elements of the FEC Framework Configuration
information for this scheme are as follows:
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Internet-Draft Raptor FEC Scheme July 2008
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Number (SBN) | Encoding Symbol ID (ESI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length (SBL) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Repair FEC Payload ID
Source Block Number (SBN), (16 bits) An integer identifier for the
source block that the repair symbols within the packet relate to.
Encoding Symbol ID (ESI), (16 bits) Integer identifier for the
encoding symbols within the packet.
Source Block Length (SBL), (16 bits) The number of source symbols in
the source block.
The interpretation of the Source Block Number, Encoding Symbol
Identifier and Source Block Length is defined by the FEC Code
Specification.
6.3. Procedures6.3.1. Source symbol construction
This FEC Scheme uses the procedures defined in Section 5 to construct
a set of source symbols to which the FEC code can be applied. The
sender MUST allocate Source Block Numbers to source blocks
sequentially, wrapping around to zero after Source Block Number
2^16-1.
During the construction of the source block:
o the length indication, l[i], included in the Source Packet
Information for each packet shall be the transport payload length.
o the value of s[i] in the construction of the Source Packet
Information for each packet shall be the smallest integer such
that s[i]*T >= (l[i]+3).
6.3.2. Repair packet construction
The number of repair symbols contained within a repair packet is
computed from the packet length. The ESI value placed into a repair
packet is given by the following formula:
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ESI_repair = I_repair + SBL,
where I_repair is the index of the repair symbol in the sequence of
repair symbols generated according to Section 6.4, where the first
repair symbol has index 0, the second index 1 etc. and SBL is the
Source Block Length. The Source Block Length field of the Repair FEC
Payload ID field SHALL be set to the number of symbols included in
the Source Packet Information of packets associated with the source
block.
6.4. FEC Code Specification
The Raptor FEC encoder defined in [RFC5053] SHALL be used. The
source symbols passed to the Raptor FEC encoder SHALL consist of the
source symbols constructed according to Section 6.3.1. Thus the
value of the parameter K used by the FEC encoder (equal to the Source
Block Length) may vary amongst the blocks of the stream but SHALL NOT
exceed the Maximum Source Block Length signalled in the FEC Scheme-
specific information. The symbol size, T, to be used for source
block construction and the repair symbol construction is equal to the
Encoding Symbol Size signaled in the FEC Scheme Specific Information.
7. Optimised Raptor FEC Scheme for arbitrary packet flows7.1. Introduction
This section specifies a slightly modified version of the FEC Scheme
specified in Section 6 which is applicable to scenarios in which only
relatively small block sizes will be used. These modifications admit
substantial optimisations to both sender and receiver
implementations.
In outline, the modifications are:
All source blocks within a stream are encoded using the same
source block size. Code shortening is used to encode blocks of
different sizes. This is achieved by padding every block to the
required size using zero symbols before encoding. The zero
symbols are then discarded after decoding. The source block size
to be used for a stream is signalled in the Maximum Source Block
Size field of the scheme-specific information. This allows for
efficient parallel encoding of multiple streams.
A restricted set of possible source block sizes is specified.
This allows explicit operation sequences for encoding the
restricted set of block sizes to be pre-calculated and embedded in
software or handware.
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This scheme is equivalent to that specified in [DVB AL-FEC
Specification] for arbitrary packet flows.
7.2. Formats and Codes7.2.1. FEC Framework Configuration Information7.2.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the fully-specified FEC scheme
defined in this section MUST be TBD as assigned by IANA.
7.2.1.2. FEC Scheme specific information
See . (Section 6.2.1.2)
7.2.2. Source FEC Payload ID
See . (Section 6.2.2)
7.2.3. Repair FEC Payload ID
SeeSection 6.2.37.3. Procedures7.3.1. Source symbol construction
See Section 6.3.17.3.2. Repair packet construction
The number of repair symbols contained within a repair packet is
computed from the packet length. The ESI value placed into a repair
packet is given by the following formula:
ESI_repair = I_repair + MSBL
Where I_repair is the index of the repair symbol in the sequence of
repair symbols generated according to Section 6.4, where the first
repair symbol has index 0, the second index 1 etc. and MSBL is the
Maximum Source Block Length signalled in the FEC Scheme Specific
Information. The Source Block Length field of the Repair FEC Payload
ID field SHALL be set to the number of symbols included in the Source
Packet Information of packets associated with the source block.
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Internet-Draft Raptor FEC Scheme July 20087.4. FEC Code Specification
The Raptor FEC encoder defined in [RFC5053] SHALL be used. The
source symbols passed to the Raptor FEC encoder SHALL consist of the
source symbols constructed according to Section 6.3.1 extended with
zero or more padding symbols such that the total number of symbols in
the source block is equal to the Maximum Source Block Length signaled
in the FEC Scheme Specific Information. Thus the value of the
parameter K used by the FEC encoded is equal to the Maximum Source
Block Length for all blocks of the stream. Padding symbols shall
consist entirely of bytes set to the value zero. The symbol size, T,
to be used for source block construction and the repair symbol
construction is equal to the Encoding Symbol Size signaled in the FEC
Scheme Specific Information. The parameter T shall be set such that
the number of source symbols in any source block is at most KMAX =
8192. The Maximum Source Block Length parameter - and hence the
number of symbols used in the FEC Encoding and Decoding operations -
SHALL be set to one of the following values:
101, 120, 148, 164, 212, 237, 297, 371, 450, 560, 680, 842, 1031,
1139, 1281
8. Raptor FEC Scheme for a single sequenced flow8.1. Formats and codes8.1.1. FEC Framework Configuration Information8.1.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the fully-specified FEC scheme
defined in this section MUST be TBD as assigned by IANA.
8.1.1.2. Scheme-specific elements
See Section 6.2.1.28.1.2. Source FEC Payload ID
The Source FEC Payload ID field is not used by this FEC Scheme.
Source packets are not modified by this FEC Scheme.
8.1.3. Repair FEC Payload ID
The Repair FEC Payload ID format for this FEC Scheme is shown below:
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Sequence Number | Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Repair FEC Payload ID
Initial Sequence Number (Flow i ISN) - 16 bits This field specifies
the lowest 16 bits of the sequence number of the first packet to
be included in this sub-block. If the sequence numbers are
shorter than 16 bits then the received Sequence Number SHALL be
logically padded with zero bits to become 16 bits in length
respectively.
Encoding Symbol ID (ESI) - 16 bits This field indicates which repair
symbols are contained within this repair packet. The ESI provided
is the ESI of the first repair symbol in the packet.
Source Block Length (SBL) - 16 bits This field specifies the length
of the source block in symbols.
8.2. Procedures8.2.1. Source symbol construction
This FEC Scheme uses the procedures defined in Section 5 to construct
a set of source symbols to which the FEC code can be applied. The
sender MUST allocate Source Block Numbers to source blocks
sequentially, wrapping around to zero after Source Block Number
2^16-1.
During the construction of the source block:
o the length indication, l[i], included in the Source Packet
Information for each packet shall be dependent on the protocol
carried within the transport payload. Rules for RTP are specified
below.
o the value of s[i] in the construction of the Source Packet
Information for each packet shall be the smallest integer such
that s[i]*T >= (l[i]+3)
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Internet-Draft Raptor FEC Scheme July 20088.2.2. Derivation of Source FEC Packet Identification Information
The Source FEC Packet Identification Information for a source packet
is derived from the sequence number of the packet and information
received in any Repair FEC packet belonging to this Source Block.
Source blocks are identified by the sequence number of the first
source packet in the block. This information is signaled in all
Repair FEC packets associated with the source block in the Initial
Sequence Number field.
The length of the Source Packet Information (in bytes) for source
packets within a source block is equal to length of the payload
containing encoding symbols of the repair packets (i.e. not including
the Repair FEC Payload ID) for that block, which MUST be the same for
all repair packets. The Source Packet Information Length (SPIL) in
symbols is equal to this length divided by the Encoding Symbol Size
(which is signaled in the FEC Framework Configuration Information).
The set of source packets which are included in the source block is
determined from the Initial Sequence Number (ISN) and Source Block
Length (SBL) as follows:
Let,
I be the Initial Sequence Number of the source block
LP be the Source Packet Information Length in symbols
LB be the Source Block Length in symbols
Then, source packets with sequence numbers from I to I +LB/LP-1
inclusive are included in the source block.
Note that if no FEC Repair packets are received then no FEC decoding
is possible and it is unnecessary for the receiver to identify the
Source FEC Packet Identification Information for the source packets.
The Encoding Symbol ID for a packet is derived from the following
information:
The sequence number, Ns, of the packet
The Source Packet Information Length for the source block, LP
The Initial Sequence Number of the source block, I
Then the Encoding Symbol ID for packet with sequence number Ns is
determined by the following formula:
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ESI = ( Ns - I ) * LP
Note that all repair packet associated to a given Source Block MUST
contain the same Source Block Length and Initial Sequence Number.
8.2.3. Repair packet construction
See Section 7.3.28.2.4. Procedures for RTP source flows
In the specific case of RTP source packet flows, then the RTP
Sequence Number field SHALL be used as the sequence number in the
procedures described above. The length indication included in the
Source Packet Information SHALL be the RTP payload length plus the
length of the CSRCs, if any, and the RTP padding bytes, if any. Note
that this length is always equal to the UDP payload length of the
packet, minus 12.
8.3. FEC Code Specification
See Section 7.49. Security Considerations
For the general security considerations related to the use of FEC,
refer to [I-D.ietf-fecframe-framework].
10. Session Description Protocol (SDP) Signaling
This section provides an SDP [RFC4566] example. The following
example uses the SDP elements for FEC Framework, which were
introduced in [I-D.ietf-fecframe-sdp-elements], and the FEC grouping
semantics [RFC4756].
In this example, we have one source video stream (mid:S1) and one FEC
repair stream (mid:R1). We form one FEC group with the "a=group:FEC
S1 R1" line. The source and repair streams are sent to the same port
on different multicast groups. The repair window is set to 200 ms.
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